#define  _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
struct ListNode 
{
    int val;
    struct ListNode* next;
};


struct ListNode* reverseList(struct ListNode* head)
{
    if (head == NULL)
        return NULL;
    struct ListNode* n1, * n2, * n3;
    n1 = NULL;
    n2 = head;
    n3 = n2->next;
    while (n2)
    {
        n2->next = n1;
        n1 = n2;
        n2 = n3;
        if (n3)
            n3 = n3->next;
    }
    return n1;
}